Low frequency tone control system



Nov. 22, 1938. A. c. sTocKER y 2,137,633

LOW FREQUENCY TONE CONTROL SYSTEM Filed March 51, 193'? FREQUENCY Gttorneg Patented Nov. 22, 1938 UNITED STATES PATENT OFFICE Arthur C. Stocker, Haddon Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 31, 1937, Serial No. 134,108

'l Claims.

. They present invention relates to a low frequency tone control system for an audio frequency amplifier or signal transmission channel, and

has for its object to provide an improved tone 5 control system of that character wherein a minimum number of low cost circuit elements are provided and which is not restricted to close tolerances in the electrical values of said circuit elements.

It is also a further object of the present invention, to provide a low frequency tone control systemfor audiov frequency amplifiers and the like, wherein, throughthe medium of a single potentiometer control means, both attenuation and augmentation of the low frequency end of the audio frequency response range of the amplifier may be provided.

It is a still further object of the present invention, to provide a low frequency tone control system which is adapted for use as a grid circuit attenuator and as part of an interstage coupling network which includes a single control means for attenuating and augmenting the low frequency portion of the response characteristic of the amplifier.

The invention will, however, be better understood from the following description, when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing,

Figure l is a schematic circuit diagram of an audio frequency amplifier having an interstage coupling network embodying the invention,

Figure 2 is a graph or vector diagram illustrating the amplitude relation between potentials developed in the coupling network of Fig. 1,

Figure 3 is a graph showing curves illustrating the operating range and tone control characteristics of the low frequency tone control network of Fig. 1, and

Figure 4 is a similar graph showing curves illustrating the low frequency response characteristic in percentage, for the limits of the control range provided by the circuit of Fig. 1.

Referring to Fig. 1, 5 and 6 are first and second stage audio frequency amplifier tubes provided with an interstage coupling means which may be of any suitable type and which, in the present example, is of the resistance coupling type hav- -ing a plate resistor 'I in the output circuit 8 of the first stage 5 and a plate supply filter comprising a series resistor element 9 and shunt bypass capacitor l0 to chassis or ground I I between 55 the plate resistor 1 and the anode supply lead,

indicated at I2. The coupling capacitor for the resistance coupling network is indicated at I3 and serves to convey signals to the grid circuit I4 of the second stage 6, through a low frequency tone control network comprising a shunt circuit ,v

connected across the audio frequency channel or output circuit of the tube 5 through said coupling capacitor.

The shunt circuit comprises two series resistors I5 and I6 with a capacitive reactance in series between them and provided by a capacitor I1. A potentiometer device I8 is connected in parallel with the capacitor I1 and the movable contact I8 thereof is connected to the grid circuit I4.

Grid bias potential is applied to the grid through a grid supply filter comprising a series filter resistor 20 connected between a bias supply lead 2| and the low potential terminal 22 of the resistor I6. An audio frequency by-pass capacitor 23-is connected between the terminal 22 and chassis or ground-.24. The audio frequency circuit is completed to cathode of the amplifier Ii through a ground connection indicated at 25.

The .tone control network comprising the resistors I5, I6 andv I8 and the capacitor I1 is an attenuator, the attenuation of which varies with frequency, and the manner of this variation being controllable by the resistor or potentiometer I8. The values of the resistors I6 and I5 and of the capacitor I'I determine the amount of low v frequency correction and the highest frequency affected. rhe control resistor or potentiometer I8 has no function other than that of control so long as its total value is several times greater than the reactance of the capacitor II at the lowest 'frequency to be considered. v

Assuming the resistances of the resistors I6, I8 and I5 tobe R4, R5 and Re respectively and the reactance of the capacitor I1 to be C3, the latter is designed to be relatively small compared to the,

resistances RL; and Re when'the applied signal frequency is above a predetermined high value, that is, above the low frequency tone control ran/ge desired. Then the voltage evERrI-Re approximately and is independent of the setting of R5, assuming that the alternating current potential applied to the tube 6 is indicated at e,

the vector sum always being equal to E. The distribution is substantially as shown in Fig. 2 wherein a and c represent the voltages across R4 and Re (R4 being somewhat larger than Re in this case), and b` represents the voltage across C3. The voltage e is then the vector sum of a and a portion of b, the portion being determined by the setting of the contact I9 on R5.

The value of e may vary from a to e5 in Fig, 2, as the potentiometer contact I9 is moved from the lower potential end of the resistor I8 through to the higher potential end thereof. With the contact I9 at the lower potential end of the tone control resistor I8, thereby including only the resistance R4 in the grid circuit, the voltage a will be equal to the Voltage e and it is obvious that the frequency characteristic of the output will drop at the lower frequency end of the audio frequency range. The ratio of the voltage e to the Voltage E is shown in Fig. 3 with respect to frequency and in this case When the contact I9 is moved to the upper or high potential end of the tone control resistor I8, it is obvious that the frequency characteristic of the output will rise at the lower frequency end of the audio frequency range. This is also shown in the curve of Fig. 3 and the voltage ratio is The low frequency limits in either case are indicated in Fig. 3 by the lines 30 and 3l and the shape of the response characteristic for attenuating the low frequency end of the audio frequency range isindicated at 32 while the augmentation of low frequencies provided is indicated by the curve 33. The normal high frequency response or limit of response is indicated by the line 34.

If the contact I9 is moved to provide the vector voltage e3 of Fig. 2, the voltage e will not be varied, by the variation in frequency of an applied signal. This may be seen by referring to Fig. 2 wherein, at infinite frequency,

and lies along the vector E. As the frequency is reduced the junction of vectors a and b moves to the right and down following a semi-circular path to at zero frequency. The junction of the vectors b and c moves to E. The vector b always crosses the vector E at the same place, the Vector b swinging around this point and the two ends of the vector b remaining always in the same ratio. If this ratio is that chosen by the contact I9 on the resistance R5, e will remain a constant portion of E regardless of frequency.

The limits of response of the low frequency control means thus provided is indicated in Fig. e by the curves 35 and 36. It is not necessary to maintain the resistance of R higher than the reactance of the tone control capacity C3 at all frequencies, although in the present example and as a preferred arrangement in the network shown, R4 and Rs are made equal to 10,000 ohms, and with a capacity at C3 of .1 mfd. the resistor R5 has a value of .5 megohm. 'I'he effect of using a lower resistance at R5 is to lessen the slope of the response characteristic at the low frequency end of the response range which gradually is reduced to zero at some very low frequency beyond the range of operation normally utilized in an audio frequency amplifier.

From the shape of the resulting curves in Figs. 3 and 4, it will be seen that the low frequency response characteristic may be varied between relatively wide limits to increase or decrease the low frequency response continuously, by operation of the single contact I9.

In any circuit, the reactance of the capacity C3 to signal frequencies above the range in which low frequency tone control is desired, is made relatively low with respect to the series resistances R4 and Re on either side of it in the shunt circuit. As has been pointed out, the resistance of the shunt potentiometer is preferably substantially greater than the reactance of the capacity C3 at the lowest frequency in the range of frequencies to be controlled.

This system permits selection of either an attenuated, a fiat, or an augmented low frequency response without recourse to resonant phenomena or complicated circuits.

When the resistors R4 and Rs in the tone control circuit are of the same order or lower than the output coupling resistor 1 in the preceding stage, the latter is unloaded as the frequency of the transmitted signals is lowered. This is for the reason that the reactance of the series capacity C3 rises, then by decreasing the shunt load created by the circuit R4-C3-Re across the resistor 1.

The effect of this is to cause E to rise and thereby augment the low frequency response.

From the foregoing description, it will be seen that a low frequency tone control network may be provided as an interstage coupling means and may be operated by a single control element to attenuate or accentuate low frequency response in a predetermined frequency range without appreciably affecting the high frequency response.

Furthermore, as above pointed out, by the variable loading effect the low frequency response may be increased automatically as the frequency of the transmitted signals is lowered.

l claim as my invention:

l. The combination with an audio frequency signal transmission circuit, of a low frequency tone control network connected in shunt with said circuit and comprising two resistance elements of the order of the resistance of the transmission circuit connected in series between the high and low potential sides of said circuit, means providing a capacity in series with and connected between said resistance elements, the capacitive reactance thereof being relatively low with respect to the resistance of said resistance elements in a higher frequency range above a predetermined upper limit of the low frequency tone control range, an output circuit, and means including a potentiometer device directly in shunt with said capacity means for variably including said capacity means in said last named circuit.

2. The combination with an audio frequency signal transmission circuit, of a low frequency tone control network connected. in shunt with said circuit and comprising two resistance elements of the order of the resistance of the transmission circuit connected in series between the high and low potential sides of said circuit, means providing a capacity in series with and connected between said resistance elements, the capacitive reactance thereof being relatively low with respect to the resistance ofA said resistance elements in a frequency range above a predetermined upper limit of the low frequency tone control range, whereby the shunt load on the transmission circuit is reduced with decreasing frequency, thereby augmenting the low frequency response.

3. The combination with an audio frequency signal transmission circuit, of a low frequency tone control network connected in shunt with said circuit and comprising two resistance elements of the order of the resistance of the transmission circuit connected in series between the high and low potential sides of said circuit, means providing a capacity in series with and connected between said resistance elements, the capacitive reactance thereof being relatively low with respect to the resistance of said resistance elements in a frequency range above a predetermined upper limit of the low frequencytone control range, a potentiometer resistor connected in shunt with said capacity means having a resistance relatively high with respect to the reactance of said capacity means at the lowest frequency in said tone control range, an output contact movable along with potentiometer resistor, and a signal output circuit connected with said contact and the low potential side of the signal transmission circuit.

4. The combination with an audio frequency signal transmission circuit, of a low frequency tone control network connected in shunt with said circuit and comprising two resist'ance elements of the order of the resistance of the transmission circuit connected in series between the high and low potential sides of said circuit, means providing a capacity in series with and connected between said resistance elements, the capacitive reactance thereof being relatively low with respect to the resistance of said resistance elements in a frequency range above a predetermined upper limit of the low frequency tone control range, an output circuit, and means including a potentiometer device directly in shunt with said capacity means for variably including said capacity means in said last named circuit, so that the output response characteristic at the low frequency end may be varied from a fraction of the input voltage indicated by E Ri-i-Rs-l-R to RVi-R E R4+R5+R where Eis the signal input voltage, e is the signal output voltage, R4 and Re are the resistance of the series connected resistance elements, and R5 is the resistance interposed across the capacitive reactance.

5. In an audio frequency amplifier, the combination with an amplifier tube having an input grid circuit, of an interstage coupling network providing tone control means in said input circuit, said network including a coupling resistor for applying audio frequency signals across said last named circuit, a pair of resistance elements having a resistance value not greater than said coupling resistor and connected in series in said last named circuit, a capacity element in series with and connected between said resistance elements, and a tone control potentiometer connected in shunt with said capacity element and having a variable output contact connected with said amplier tube to apply signals thereto.

6. In an audio frequency amplier, the combination with an amplifier tube having an input grid circuit, of an interstage coupling network providing tone control means in said input circuit, said network including a coupling resisto-r for applying audio frequency signals across said last named circuit, a pair of resistance elements having a resistance value not greater than said coupling resistor and connected in series in said last named circuit, a capacity element in series with and connected between said resistance elements providing reactance relatively low with respect to the resistance of said resistance elements in a frequency range above a predetermined low frequency, and a tone control potentiometer connected in shunt with said capacity element and having a variable output contact connected with said amplifier tube to apply signals thereto, said potentiometer having a resistance relatively high with respect to the reactance of said capacity element at the lowest frequency to be controlled.

7. In an audio frequency amplifier, the combination with an amplifier tube having an input grid circuit, of an interstage coupling network providing tone control means in said input circuit, said network including a coupling resistor for applying audio frequency signals across said last named circuit, a pair of resistance elements having a resistance value not greater than said coupling resistor connected in series in said last named circuit, a capacity element in series with and connected between said resistance elements providing reactance relatively low with respect to the resistance of said resistance elements in a frequency range above a predetermined low frequency, and a tone control potentiometer connected in shunt with said capacity element havlng a variable output contact connected with said amplier tube to apply signals thereto, said potentiometer having a resistance relatively high with respect to the reactance of said capacity element at the lowest frequency to be controlled, and means providing' a low impedance signal path to ground each for the low potential end of said coupling resistor and one of said series connected resistors.

ARTHUR C. STOCKER. 

